Conventionally, the interlayer connections in a multilayer printed circuit board are formed by drilling down one or more via holes through a dielectric layer of film to the next underlying circuit layer and filling the resulting via holes with a conductive material to connect two vertically isolated conductive layers. This process is repeated on one side of the insulation substrate to form a multilayer printed circuit board.
U.S. Pat. No. 5,092,032 discloses a method for manufacturing multilayer printed circuit boards which does not require the step of drilling through the dielectric layer and in which landless inter-layer connection is made between a lower-layer electric circuit and an upper-layer electric circuit formed on a substrate. This method, which utilizes a single photoresist to form a circuitization layer and the conductive via extending upwardly from the circuitization layer, involves forming a via bump in a photoresist hole. Such a via bump is suitable for transmitting power; it may experience at least some inconvenience in transmitting signals.
U.S. Pat. No. 5,451,721 discloses a method for manufacturing multilayer printed circuit by electrically connecting power conductors or ground conductors using through holes. Signal conductors in any two adjacent signal wiring layers are electrically connected using via holes extending only through an intervening electrically insulating layer. One of its limitations is that the electrically insulating layer must be a layer of photosensitive resin so that the via holes can be formed using conventional lithophotographic techniques.